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June 15 is now the target date for completing the replacement of I-85's damaged sections, state transportation Commissioner Russell McMurry said Tuesday -- sooner than previous estimates of at least several months.

He called it a "very aggressive but attainable date" and "much better than our initial thought."

State will review storage policies

McMurry said the state will review its policies on storing materials under bridges and elevated highways -- an issue in this case because authorities say last Thursday's fire started where the state kept high-density plastic conduits and other construction materials.

The state was storing high-density polyethylene (HDPE) pipes and fiberglass conduits under the I-85 overpass, and the fire spread to the pipes, McMurry said. Authorities say they've arrested a man accused of intentionally setting the fire.

The material was in a state-owned lot under the overpass, behind a chain-linked fence with a locked gate, with a "no trespassing" sign, McMurry said.

The storage is not believed to have broken any current state policy, but McMurry said lessons could be learned -- especially regarding whether the material was stored securely enough.

"Obviously, what we know now, if we knew then, we would not have the material stored in the fashion that it was stored," he said, responding to question about how secure the site was. "The gating and the fencing was really to keep the material from being stolen or removed from the area."

McMurry said he already sent a letter to other states' department of transportation officials "to make sure they may want to make a full assessment" about what they are storing under their bridges and how they are doing it.

"Hopefully, (there will) be lessons learned, not only for Georgia, but the nation," he said of the coming review.

How the fire, collapse happened

The fire started under I-85 in northeast Atlanta, north of the highway's split with I-75.

At first, I-85 motorists drove through the smoke, and firefighters fought the flames below. It eventually grew into a massive fireball; the road was closed and firefighters withdrew as the structure weakened.

A portion of northbound I-85 collapsed, injuring no one. In total, about 700 feet of northbound and southbound I-85 must be replaced, officials say.

The closure is proving to be a major challenge for motorists in one of the country's most congested cities. Traffic is being diverted to already heavily traveled I-75, I-285 and surface streets.

Police arrested Basil Eleby on suspicion of setting the fire. He is charged with first-degree arson and criminal damage to property. Police also arrested two companions of his on charges of trespassing, alleging that they were with Eleby at the time; investigators say they believe all three are homeless.

According to an affidavit obtained by CNN, one of the companions "watched Basil Eleby place a chair on top of a shopping cart, reach under the shopping cart and ignited it."

Eleby told investigators that he and his companions talked about smoking cocaine under the highway overpass, according to the affidavit. He said he decided to smoke without his companions, and left before the fire started, the affidavit said.

Plastic pipes had been there since 2011

The fire eventually burned the HDPE pipes, which the state had stored there since about 2011, McMurry said.

HDPE pipes encase and protect cables, and are widely used in the transportation industry to build "smart" highways that provide information to drivers, control traffic signal lights and tollways.

Details about the fire and how it brought down the highway are under investigation.

The flammability of HDPE is relatively low, said Tony Radoszewski, president of the Plastics Pipe Institute, a trade group based in Irving, Texas. HDPE would have to be exposed to a high-temperature flame for a considerable amount of time to burn, he said.

"Somebody had to start a fire. It doesn't combust by itself, it needs fuel," Radoszewski told CNN. "Someone had to do it. It's not like someone would have dropped a match and it started."

McMurry has said that HDPE's flammability -- the ease at which something can be set on fire -- is low, and that it is "not uncommon" for states to store construction materials under bridges.

But he said Tuesday the state checked what was being stored under bridges after last week's fire, and no other bridge in Georgia had been found to have HDPE stored under it.

Why were the pipes there?

The HDPE and fiberglass conduits were meant for a project that a contractor bid for in 2007, McMurry said. The contractor was to install advanced traffic management systems -- like cameras and message boards -- along State Route 400 between I-85 and I-285, McMurry said.

But the contractor defaulted in 2008. The project eventually was done by a different contractor, who used different materials, the commissioner said.

So, GDOT, which already paid for the unused conduits, took control of them "in hopes that it could be used for another project," McMurry said.

The conduits initially were stored near an intersection of a surface street and State Route 13. By around 2011, McMurry said, they were moved to the lot under I-85, where the fire happened last week.